The invention relates to detectors for detecting differentially phase encoded radio signals.
In general, a differentially phase modulation signal detector includes a phase detector that is capable of measuring the relative phase shift between two adjacent phase-modulated symbols. This is usually performed by applying the received radio signal to a narrow band filter, after which the signal is applied to two channels, a phase detector channel and a delay channel. The phase detector channel compares the phase of a received symbol with the phase of a previously received symbol to determine the phase relationship between the two symbols. The delay channel delays a received data symbol for one data symbol period after which the prorogated symbol is applied to the phase detector for comparison with a subsequent received symbol. Based upon the information of the phase detector, a decision circuit decides, usually in the presence of noise, that a logic zero was transmitted if there is a phase shift magnitude of less than 90.degree. between the data symbols or a logic one was transmitted if there was a phase shift magnitude of greater than 90.degree.. A description of the operation of phase modulation signal detectors such as DPSK detectors is provided in "Fundamentals of Analog & Digital Communication Systems", B. Simpson and R. Houts, Allyn & Bacon, Publishers, 1971, Section 6(a) and in "Principles of Communication Systems", Electronic & Electronic Engineering Series, H. Taub and D. Schilling, McGraw-Hill, 1971, pages 224 through 227.
In the case where each bit or data symbol is encoded with a changing or a pseudo-randomly changing spread spectrum chip sequence that results in a different chip code sequence for each bit or symbol, then detecting of the data becomes very difficult using the phase modulation detection technique described above.
Differentially noncoherent phase shift keying, as well as other differentially phase encoded waveforms can be detected by utilizing sample data techniques. Differentially modulation signal detectors have been built for detecting spread spectrum signals using matched filter correlators such as Surface Acoustic Wave devices (SAWD's), Fast Fourier Transform (FFT) processors, and Inductive Capacitive (IC) matched filters. However, there are many trade-offs associated with the above devices when used in the detection of differentially encoded waveforms at baseband. Recent developments (as disclosed in "Speed of CCD Memory is Boosted Tenfold", Electronic Design, Vol. 23, No. 26, Dec. 20, 1975, page 16) in the technology of Charge Transfer Devices (CTD) indicate that the transfer rate of CTD's is sufficiently high enough to be used for baseband detection of signals.
There was disclosed in "Principles of Communications Systems", Electronic & Electronic Engineering Series, H. Taub & Schilling, 1971, McGraw-Hill, pages 383 through 385 a common realization of a noncoherent matched filter or noncoherent digital matched filter that was used in conjunction with low pass quadrature techniques and which was matched to a signal. The article focused on the special case of binary signals (one bit digitalizations) and a low pass realization of signals using low pass quadrature techniques.
The use of surface acoustic wave devices as convolvers for DPSK demodulation of spread spectrum signals is covered in an article by S. A. Reible, et al. published in IEEE Proceedings of the 1976 Sonics & Ultrasonics Symposium, entitled "Convolvers for DPSK Demodulation of Spread Spectrum Signals" which provides details on the design and performance of an acoustical electric convolver developed for decoding differential phase shift keyed data at 100 KBits/second rate. The data is encoded on 100 M chips/second pseudo-noise spread spectrum carrier. A 20 microsecond convolver, segmented at its center, acts as an electronically programmable matched filter, providing simultaneous correlated spikes for two adjacent 10 microsecond bit waveforms. Subsequent comparison using a sum and difference hybrid and an envelope detector allows for a decision as to the presence of a phase reversal between two data bits.